The sensitivity of an imaging element (such as CCD or CMOS) for e.g. a camera, or a light receiving element for e.g. an automatic exposure meter, covers from the visible light region to the near infrared region. On the other hand, the spectral sensitivity of the human eyes is limited to the visible light region. Therefore, for example, in the case of a camera, the sensitivity of its imaging element is corrected to be close to the spectral sensitivity of the human eyes, by providing a near infrared filter to transmit light in the visible light region (from 420 to 630 nm) and to absorb or reflect light in the near infrared region (from 700 to 1,100 nm) between a lens and the imaging element. In order to bring the sensitivity closer to the spectral sensitivity of the human eyes, the near infrared filter is required to be such that the transmittance sharply changes in the wavelength range of from 630 to 700 nm.
As a near infrared filter, one having a near-infrared-absorbing coating film is known which is formed by coating the surface of a glass substrate with a coating material having e.g. a binder resin added to a dispersion having near-infrared-absorbing particles dispersed in a dispersion medium.
Further, as such near-infrared-absorbing particles, some which contain copper and phosphoric acid have been proposed.
(1) Near-infrared-absorbing particles to absorb light with a wavelength of from 700 to 1,100 nm, which were obtained by treating, with an aluminum compound, the surface of near-infrared-absorbing particles wherein the molar ratio of CuO/P2O5 is from 0.05 to 4 when copper is calculated as CuO, and phosphoric acid is calculated as P2O5 (Patent Document 1).
(2) A dispersion having copper phosphate dispersed in a dispersing medium by means of a dispersant (Patent Document 2).
It has been confirmed that the near-infrared-absorbing particles (1) and the dispersion (2), as well as a near-infrared-absorbing coating film formed by using them, absorb near infrared rays with a wavelength of at least 800 nm. However, such a near-infrared-absorbing coating film does not sufficiently satisfy the performance required for a near infrared filter, as the transmittance does not sharply change in the wavelength range of from 630 to 700 nm.